Method and apparatus for improved detection of rate errors in variable rate receivers

ABSTRACT

A system and method for detection of rate determination algorithm errors in variable rate communications system receivers. The disclosed embodiments prevent rate determination algorithm errors from causing audible artifacts such as screeches or beeps. The disclosed system and method detects frames with incorrectly determined data rates and performs frame erasure processing and/or memory state clean up to prevent propagation of distortion across multiple frames. Frames with incorrectly determined data rates are detected by checking illegal rate transitions, reserved bits, validating unused filter type bit combinations and analyzing relationships between fixed code-book gains and linear prediction coefficient gains.

CLAIM OF PRIORITY UNDER 35 U.S.C. §120

The present Application for Patent is a Continuation and claims priorityto U.S. patent application Ser. No. 09/730,147, filed Dec. 4, 2000, nowgranted, entitled “METHOD AND APPARATUS FOR IMPROVED DETECTION OF RATEERRORS IN VARIABLE RATE RECEIVERS,” and a Continuation of U.S. patentapplication Ser. No. 10/938,445, filed Sep. 9, 2004, now allowed,entitled “METHOD AND APPARATUS FOR IMPROVED DETECTION OF RATE ERRORS INVARIABLE RATE RECEIVERS.” The foregoing assigned applications areassigned to the assignee hereof and hereby expressly incorporated hereinby reference in their entirety.

BACKGROUND

I. Field

The disclosed embodiments relate to wireless communications. Moreparticularly, the disclosed embodiments relate to a novel and improvedmethod and apparatus for detecting, at a receiver of a variable ratecommunication system, errors in the determination of the rate at whichdata has been encoded for transmission.

II. Background

FIG. 1 is an illustrative stepdiagram of a variable rate CDMAtransmission system 10 described in the Telecommunications IndustryAssociation over-the-air interface standard TIA/EIA Interim Standard 95,and its derivatives, such as, e.g., IS-95B (hereinafter referred tocollectively as IS-95). This transmission system may be provided, forexample, within a base station of a cellular transmission system for usein transmitting signals to mobile telephone subscriber units within acell surrounding the base station. It may also be provided within mobiletelephone subscriber units for use in transmitting signals to a basestation.

A microphone 11 detects a speech signal which is then sampled anddigitized by an analog to digital converter (not shown). A variable ratedata source 12 receives the digitized samples of the speech signal andencodes the signal to provide packets of encoded speech of equal framelengths. Variable rate data source 12 may, for example, convert thedigitized samples of the input speech to digitized speech parametersrepresentative of the input voice signal using Linear Predictive Coding(LPC) techniques. In an exemplary embodiment, the variable rate datasource is a variable rate vocoder as described in detail in U.S. Pat.No. 5,414,796, which is assigned to the assignee of the presentinvention and is incorporated by reference herein. Variable rate datasource 12 provides variable rate packets of data at four possible framerates 9600 bits per second (bps), 4800 bps, 2400 bps, and 1200 bps,referred to herein as full, half, quarter, and eighth rates. Packetsencoded at full rate contain 172 information bits, packets encoded athalf rate contain 80 information bits, packets encoded at quarter ratecontain 40 information bits, and packets encoded at eighth rate contain16 information bits. Packet formats are shown in FIGS. 2A-2D. Thepackets, regardless of size, all are one frame length in duration, i.e.20 ms. Herein, the terms “frame” and “packet” may be usedinterchangeably.

The packets are encoded and transmitted at different rates to compressthe data contained therein based, in part, on the complexity or amountof information represented by the frame. For example, if the input voicesignal includes little or no variation, perhaps because the speaker isnot speaking, the information bits of the corresponding packet may becompressed and encoded at eighth rate. This compression results in aloss of resolution of the corresponding portion of the voice signal but,given that the corresponding portion of the voice signal contains littleor no information, the reduction in signal resolution is not typicallynoticeable. Alternatively, if the corresponding input voice signal ofthe packet includes much information, perhaps because the speaker isactively vocalizing, the packet is encoded at full rate and thecompression of the input speech is reduced to achieve better voicequality.

This compression and encoding technique is employed to limit, on theaverage, the amount of information being transmitted at any one time tothereby allow the overall bandwidth of the transmission system to beutilized more effectively to allow, for example, a greater number oftelephone calls to be processed at any one time.

The variable rate packets generated by data source 12 are provided topacketizer 13, which selectively appends Cyclic Redundancy Check (CRC)bits and tail bits. As shown in FIG. 2A, when a frame is encoded by thevariable rate data source 12 at full rate, packetizer 13 generates andappends twelve CRC bits and eight tail bits. Similarly, as shown in FIG.2B, when a frame is encoded by the variable rate data source 12 at halfrate, packetizer 13 generates and appends eight CRC bits and eight tailbits. As shown in FIG. 2C, when a frame is encoded by the variable ratedata source 12 at quarter rate, packetizer 13 generates and appendseight tail bits. As shown in FIG. 2D, when a frame is encoded by thevariable rate data source 12 at eighth rate, packetizer 13 generates andappends eight tail bits.

The variable rate packets from packetizer 13 are then provided toencoder 14, which encodes the bits of the variable rate packets forerror detection and correction purposes. In an exemplary embodiment,encoder 14 is a rate 1/3 convolutional encoder. The convolutionallyencoded symbols are then provided to a CDMA spreader 16, animplementation of which is described in detail in U.S. Pat. Nos.5,103,459 and 4,901,307. CDMA spreader 16 maps eight encoded symbols toa 64-bit Walsh symbol and then spreads the Walsh symbols in accordancewith a pseudo-random noise (PN) code.

Repetition generator 17 receives the spread packets. For packets of lessthan full rate, repetition generator 17 generates duplicates of thesymbols in the packets to provide packets of a constant data rate. Whenthe variable rate packet is half rate, the repetition generator 17introduces a factor of two redundancy, i.e., each spread symbol isrepeated twice within the output packet. When the variable rate packetis quarter rate, the repetition generator 17 introduces a factor of fourredundancy. When the variable rate packet is eighth rate, the repetitiongenerator 17 introduces a factor of eight redundancy.

Repetition generator 17 provides the aforementioned redundancy bydividing the spread data packet into smaller sub-packets referred to as“power control groups.” In the exemplary embodiment, each power controlgroup comprises 6 PN spread Walsh Symbols. The constant rate frame isgenerated by consecutively repeating each power control group therequisite number of times to fill the frame as described above.

The spread packets are then provided to a data burst randomizer 18,which removes the redundancy from the spread packets in accordance witha pseudo-random process as described in U.S. Pat. No. 5,535,239,assigned to the assignee of the present invention. Data burst randomizer18 selects one of the spread power control groups for transmission inaccordance with a pseudo-random selection process and gates the otherredundant copies of that power control group.

The packets are provided by data burst randomizer 18 to finite impulseresponse (FIR) filter 20, an example of which is described in U.S. Pat.No. 5,659,569, and assigned to the assignee of the present invention.The filtered signal is then provided to digital to analog converter 22and converted to an analog signal. The analog signal is then provided totransmitter 24, which up-converts and amplifies the signal fortransmission through antenna 26.

FIG. 3 illustrates pertinent components of a base station. In anotherembodiment, the apparatus of FIG. 3 could reside in a mobile telephone28 or other mobile station receiving the transmitted signal. The signalis received by antenna 30, down-converted and amplified, if necessary,by receiver 32. The signal is then provided to frame rate detection unit33, which subdivides the signal into packets and determines thecorresponding frame rate for each packet. The frame rate may bedetermined, depending upon the implementation, by detecting the durationof individual bits of the frame. The packet and a signal identifying thedetected frame rate for the packet are then forwarded to CRC unit 34 forperforming cyclic redundancy checks or related error detection checks inan attempt to verify that no transmission errors or frame rate detectionerrors occurred. A frame rate detection error results in the packetbeing sampled at an incorrect rate resulting in a sequence of bits thatare effectively random. A transmission error typically results in onlyone or two bits being in error. Usually, if a transmission error orframe rate detection error occurs, the CRC unit detects the error. “Bad”frames failing the CRC are erased or otherwise discarded by frameerasure unit 36. “Good” frames which pass the CRC are routed to variablerate decoder 40 for conversion back to digitized voice signals. Thedigitized voice signals are converted to analog signals by a digital toanalog converter (not shown) for ultimate output through a speaker 42 ofthe mobile telephone.

Depending upon the implementation, no separate frame erasure unit 36 isnecessarily required. Rather, CRC unit 34 may be configured merely tonot output bad frames to variable rate decoder 40. However, provision ofa frame erasure unit facilitates generation of frame erasure signals forforwarding back to the base station to notify the base station of theframe erasure error. The base station may use the frame erasureinformation to modulate the amount of power employed to transmitsignals, perhaps as part of a feedback system intended to minimizetransmitted power while also minimizing frame errors.

As noted above, by varying the frame rate of packets to thereby compressthe information contained therein, the overall bandwidth of the systemis utilized more effectively, usually without any noticeable effect onthe transmitted signal. However, problems occur occasionally which havea noticeable effect. One such problem occurs if a frame subject to aframe rate detection error or a transmission error nevertheless passesthe CRC. In such cases, the bad frame is not erased but is processedalong with other good frames. The error may or may not be noticeable.For example, if the error is a transmission error wherein only one ortwo bits of encoded speech are in error, the error may have only anextremely slight and probably unnoticeable effect on the output voicesignal. However, if the error is a frame rate detection error, theentire packet will thereby be processed using the incorrect frame ratecausing effectively random bits to be input to the decoder, likelyresulting in a noticeable artifact in the output voice signal.Noticeable artifacts caused by frame rate detection errors are suchunacceptable distortions as screeches, or beeps. For some systems, ithas been found that incorrect frame rate detections occur with aprobability of about 0.005% yielding an incorrectly received packet anda corresponding artifact in the output voice signal about every sixteenminutes of conversation time. Although described with respect to a CDMAsystem using IS-95 protocols, similar problems can occur in almost anytransmission system employing variable transmission rates and in relatedsystems as well.

Due to effects of channel conditions such as noise, and multi-pathfading on received parameters, rate determination algorithms (RDA) offrame rate detection units 33 do not guarantee that the received framerate is correct. Given that this is a limitation of the RDA, it isdesirable to ensure that such RDA errors don't cause audible artifactssuch as screeches, or beeps. When the received frame is not suitable foraccurate rate-determination due to poor channel conditions, the RDAeither determines that the frame has to be erased or it assigns anincorrect rate to the packet. Typically, the speech decoder has aframe-erasure processing mechanism that perceptually smooths the lostframes using past frames, and produces speech that is not annoying tothe listener. However, if instead of a frame erasure, the RDA assigns anincorrect rate to the frame, random bits are fed into the variable ratedecoder 40. Unless detected, the random bits can produce very loud,annoying artifacts such as screeches, and beeps. It is generally truethat a frame-erasure does not produce as much speech qualitydegradations as an incorrect rate frame.

It is desirable to handle these incorrect rate frames without generatingaudible artifacts. It is desirable to detect an incorrect rate frame,and perform frame-erasure processing, and/or clean-up the memory statesin the variable rate decoder 40, such that effects of incorrect ratedetermination do not propagate across many frames.

Therefore, it can be appreciated that there is a significant need for amethod that detects rate determination errors in a wirelesscommunication system, and eliminates resultant audible artifacts.

SUMMARY

The disclosed embodiments are directed to a system and method fordetection of rate determination algorithm errors in variable ratecommunications system receivers. Accordingly, a method for detectingrate errors in a variable rate receiver, comprising receiving an encodedspeech signal, performing a rate determination algorithm on the speechsignal to provide an encoded rate, and detecting errors in the providedrate, is described.

In another embodiment, A rate error detection system, comprising areceiver for receiving an encoded speech signal, a rate determinationelement for performing a rate determination algorithm on the speechsignal to provide an encoded rate, and a rate error detector fordetecting errors in the provided rate, is described.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the disclosed embodiments willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a is a stepdiagram of a conventional transmit portion of adigital cellular telephone system base station;

FIGS. 2A-2D are illustrations of conventional frame formats employed bythe system of FIG. 1;

FIG. 3 is a stepdiagram of a conventional receive portion of a cellulartelephone, configured without the disclosed embodiments, for receivingsignals transmitted by the system of FIG. 1;

FIG. 4 is a stepdiagram of a receive portion of a mobile subscriberunit, configured in accordance with the disclosed embodiments of a rateerror detector, for receiving signals transmitted by the system of FIG.1;

FIG. 5 is a flowchart diagram of a method for detection of rate errorsin frames identified as full rate frames;

FIG. 6 is a flowchart diagram of a method for detection of rate errorsin frames identified as half rate frames;

FIG. 7 is a flowchart diagram of a method for detection of rate errorsin frames identified as quarter rate frames;

FIG. 8 is a flowchart diagram of a method for detection of rate errorsin frames identified as eighth rate frames; and

FIG. 9 is graph illustrating an exemplary fixed codebook gain vs. LPCthreshold curve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exemplary embodiment of improved detection of rate errors in variablerate receivers is implemented in a Selectable Mode Vocoder (SMV). TheSMV is a variable rate vocoder, and is a candidate to be used by thethird generation CDMA system, IS2000. The SMV vocoder algorithm uses avariety of parameters such as source-controlled rate, frame-type, LPcoefficients, adaptive, and fixed code-book parameters. Speech to beencoded is analyzed for the amount of perceptual information itcontains. The analysis classifies speech into various types such asbackground noise, stationary unvoiced speech, stationary voiced speech,and non-stationary speech (onsets, transients, etc.). Inter-speechbackground noise is encoded using eighth rate. Stationary unvoicedspeech is encoded using a quarter-rate Noise Excited Linear Prediction(NELP) scheme. Stationary voiced speech is encoded using a full or halfrate Type-1 CELP scheme. Non-stationary speech is encoded using a fullor half rate Type-0 CELP scheme. The Type information controls severalaspects of the coding of the frame, such as the sub-frame size, theparameters used for speech representation, and the coding scheme forthese parameters. Frames of type 0 are “non-periodic” frames, where thetypical parameters, such as pitch correlation and pitch lag, can changerapidly. Thus, in Type-0 CELP the pitch lag is coded and transmittedmore frequently (i.e., for every sub-frame). Frames of Type-1 are“periodic” frames that have high periodicity and are perceptually wellrepresented with a smooth pitch track. In Type-1 CELP the pitch lag iscoded once per frame, and the interpolated pitch track is derived fromthis lag. Because of the high periodicity and the smooth pitch track,the pitch gains exhibit very stable behavior and are jointly quantized.One bit of each stationary voiced and non-stationary speech frame isused to indicate the CELP scheme type.

One skilled in the art would recognize that the SMV may be implementedusing field-programmable gate arrays (FPGAs), programmable logic devices(PLDs), digital signal processors (DSPs), one or more microprocessors,an application specific integrated circuit (ASIC), or any other devicecapable of performing the SMV functions described above.

The disclosed embodiments are described in the context of CDMA phones.However, it should be understood that the disclosed embodiments areapplicable to other types of communications systems and modulationtechniques, such as Personal Communications Systems (PCS), wirelesslocal loop (WLL), private branch exchange (PBX), or other known systems.Furthermore, systems utilizing other well known transmission modulationschemes such as TDMA and FDMA as well as other spread spectrum systemsmay employ the disclosed embodiments.

In accordance with one embodiment, FIG. 4 illustrates pertinentcomponents of a mobile subscriber unit 28, or other mobile station,receiving a signal provided by a base station transmission system suchas the system of FIG. 1 wherein a signal having variable rate packets istransmitted. Frame rates include full rate, half rate, quarter rate, andeighth rate as shown in FIGS. 2A-2D. The packets include encoded speechparameters representative of a compressed voice signal. In addition,each packet includes CRC bits and/or encoder tail bits. Additionaldetails regarding the content of the packets is provided above inconnection with FIG. 1 and in U.S. Pat. No. 5,414,796 referenced above.

The illustrated components of FIG. 4 are similar to those of FIG. 3 andonly pertinent differences will be described in detail. The transmittedsignal is received by antenna 30, and downconverted and amplified byreceiver 32. The signal is then provided to a frame rate detection unit33, which attempts to determine the corresponding frame rate for thepacket, using a Rate Determination Algorithm (RDA). The packet is thenprovided to a CRC unit 34 for performing cyclic redundancy checks onframes of the received signal in an attempt to verify that no frame ratedetection error or transmission error occurred. Frames failing the CRC,i.e. bad frames, are erased by frame erasure unit 36. As noted above, noseparate frame erasure unit is necessarily required. Rather, framessubject to CRC errors may merely not be output from the CRC unit 34. Ineither case, frames that pass the CRC, i.e. potentially good frames, arerouted to a rate error detector 38. Depending upon the implementation,no separate rate error detector unit 38 is necessarily required. Rather,rate error detector unit 38 may be implemented in the SMV or integratedwith other receiver components.

The rate error detector 38 further examines the frames to verify thatthe frame rate detected by the RDA of frame rate detection unit 33 isindeed correct. The frames are further verified by rate error detector38 using the verification methods for full, half, quarter, and eighthrate frames described below in detail with reference to FIGS. 5-8.Frames failing verification may be erased by frame erasure unit 36.Frames failing verification may also be processed to clean up memorystates in the variable rate decoder 40 so that distortion does notpropagate across many frames. Rate, control, and frame information isoutput from the rate error detector 38 to the variable rate decoder 40for cleanup processing. Frames that pass the rate error detectorverification are routed directly to variable rate decoder 40.

Variable rate decoder 40 processes the frames by decoding speechparameters contained therein for conversion back to digitized voicesignals. The digitized voice signals are ultimately converted to analogsignals by a digital to analog converter (not shown) for output througha speaker 42 of the to a listener when the receiver is a mobilesubscriber unit. The digital signal may be further propagated within awireless system when the receiver is a base station.

FIGS. 5-8 describe in detail the frame rate verification methods inaccordance with the embodiments for full, half, quarter, and eighth rateframes performed by the rate error detector (FIG. 4, element 38). Theverification methods employ novel use of illegal classificationtransitions for frames of specified rates and types, reserved bitchecking, illegal filter type verification, and analysis of fixedcodebook (FCB) vs. LPC threshold gain curves. Additionally, thedisclosed embodiments of FIGS. 5-8 employ novel use of frame erasureprocessing and memory state manipulation to smooth effects of detectedframe rate errors.

The disclosed embodiments impose a novel state transition structure onrate transitions of consecutive frames based on knowledge of speechclassification and the phonetic character of conversational speech. Ratetransitions that violate the structure are illegal, and are used todetect frames rate errors. These illegal rate transitions are defined tocomprise:

a full-rate frame followed by an eighth-rate frame;

a full-rate, Type-1 frame followed by an eighth-rate frame;

a half-rate, Type-1 frame followed by an eighth-rate frame;

a quarter-rate frame followed by a Type-1 full-rate frame;

a quarter-rate frame followed by a Type-1 half-rate frame;

an eighth-rate frame followed by a Type-1 full-rate frame;

an eighth-rate frame followed by a Type-1 half-rate frame;

an eighth-rate frame followed by a quarter-rate frame followed by aneighth-rate frame;

an eighth-rate frame followed by a half-rate frame followed by aneighth-rate frame; and

an eighth-rate frame followed by a full-rate frame followed by aneighth-rate frame.

Based on the current and previous frame rates and types, the presence ofillegal transitions indicates an RDA error in either the current frameor the previous frame.

The disclosed embodiments employ novel use of full rate and quarter ratereserved transmitted bits for detecting RDA errors. A full-rate packethas 171 information bits per 20 ms frame, in which 1 bit is a reservebit. The reserve bit can be set by an encoder to a fixed value of eitherzero or one. The reserved bit is checked by the rate error detector(FIG. 4, element 38) to determine whether the received reserve bit hasthe expected fixed encoded value. Reserved bits not received as expectedindicate an RDA error in the current full rate frame. A quarter-ratepacket has 40 bits per 20 ms frame in which NELP uses 39 bits, while onebit is unused. Again, the unused bit can be set by the encoder to afixed value of either zero or one. The unused bit is checked by the rateerror detector (FIG. 4, element 38) to determine whether the receivedunused bit has the expected fixed encoded value. Unused bits notreceived as expected indicate an RDA error in the current quarter rateframe.

The disclosed embodiments employ novel use of illegal filter typechecking for quarter rate NELP frames to detect rate errors. NELPencoding employs spectral shaping of pseudo-random excitation using oneof 3 different shaping filters. Two bits are used to transmit the indexof the selected filter. Three of the two-bit patterns are used toidentify the selected shaping filter, leaving a fourth 2-bit patternunused, or illegal. Presence of the unused, or illegal pattern indicatesan RDA error in the current quarter rate NELP frame.

The disclosed embodiments employ novel use of encoded parameters fordetecting rate errors. Investigations into effects of RDA errors onvocoders reveal that audio artifacts such as screeches and beeps aremainly caused by excessively high FCB gain values accompanied by highLPC prediction gain values. Natural speech, when analyzed by an encoderfor encoding parameters, produces FCB gains and LPC prediction gainsthat have an inverse relation with respect to each other. In otherwords, when the LPC gain is high, the FCB gains is generally low, andwhen the LPC gain is low, the FCB gain is generally high.

The inverse relationship of FCB gains and LPC gains in natural speechproduce a curve in the graph of FCB gain vs. LPC gain above which thereis no representation of good natural speech. The FCB gain, and hence thegraphical curve can be a function of the input speech level. Framesreceived at levels above the curve, where there is no representation ofgood natural speech indicate a rate error in the frame. A novel methodfor removing the variance due to the input level, when a rate error isgraphically detected, is to normalize the FCB gain using an averageenergy value computed from past frames. FIG. 9 shows a scatter plotindicating the relationship between the normalized FCB gain and the LPprediction gain. Circles below the solid curve were generated by cleanspeech, and the asterisks above the solid lines correspond tounacceptable screeches caused by RDA errors. The solid curve representsa threshold curve that separates the region of good speech fromunacceptable screeches or other artifacts. This threshold can easily berepresented in a parametric form and incorporated into the rate errordetector (FIG. 4, element 38). After the FCB gain and the LPC gains havebeen established for a received packet, a check can be performed todetermine if the frame lies below the threshold curve. If the frame doesnot lie below the threshold cure, screeching due to an RDA error isindicated.

The disclosed embodiments employ novel use of eighth rate excitationgain to detect rate errors. Because the eighth-rate coding scheme isused only for background noise portions of speech, the excitationenergy, which is quantized using a gain parameter, has an upper limit.When the excitation gain is obtained from the received packets, a checkcan be performed to see if the excitation gain lies below the gainparameter upper limit. An RDA error is indicated if the gain parameterdoes not lie below the upper limit.

When any of the disclosed detection mechanisms indicates a frame rateerror in either the current frame or immediately preceding frames, thedisclosed embodiments may employ one or more novel schemes whiledecoding to eliminate the resultant distortion and/or prevent thedistortion from propagating across multiple frames. The schemes compriseframe erasure processing, reduction of FCB gain, and resetting of memorystates.

Vocoders typically have a built-in process to handle frame erasures. Theframe erasure process can be used by the disclosed embodiments for anyframe in which the rate error detector (FIG. 4, element 38) has detectedan RDA error. The frame erasure process synthesizes speech without usingany information from the current frame, and derives all the decoderparameters from the past memory in order to produce speech that isperceptually smoothed with respect to the previous frames.

When a rate error is detected because a frame is received at a levelabove the FCB vs. LPC gain curve where there is no representation ofgood natural speech, the decoder (FIG. 4, element 40) can forciblyreduce the FCB gain to a lesser value that will prevent a high energyscreech or beep from being produced at the output of the decoder (FIG.4, element 40).

Vocoder algorithms typically reconstruct speech using past memorystates. These memory states include the Moving-average vector quantizer(MAVQ) memory for FCB gain, excitation memory, LPC synthesis memory, andpost-filter synthesis memory. An undetected RDA error can inject badvalues into these memories. The effects of bad values can last for manyframes in the future, even if all the future frames are healthy frames.To prevent corruption of future frames RDA errors detected as describedin the current or immediately previous frames, can cause the FCB gainMAVQ, Excitation, LPC synthesis filter, and Post-filter synthesis filtermemories to be reset to predefined values that do not produce highenergy screeches. In one embodiment, the memory values are overwrittenwith zeros. In another embodiment, the memory values are overwrittenwith their respective initialization values.

FIG. 5 illustrates a method in accordance with one embodiment fordetection of rate errors in frames identified by the RDA as full rateframes. One skilled in the art will understand that the ordering ofsteps illustrated in FIG. 5 is not limiting. The method is readilyamended by omission or re-ordering of the steps illustrated withoutdeparting from the scope of the disclosed embodiments.

In step 502, the rate error detector inputs a data frame determined bythe RDA to be a full rate frame. Control flows to step 504.

In step 504, the reserved bit or sanity bit is tested to determinewhether the received value equals the fixed value set by the encoder. Ifthe bit is not equal to the fixed value set by the encoder, indicating aframe rate error, control flow proceeds to step 506. Otherwise, controlflow proceeds to step 510.

In step 506, frame erasure processing and/or memory state resetprocessing is performed. Control flow proceeds to step 508 wheredecoding continues.

In step 510, the frame is checked to determine whether it is frame type.If the frame is a Type-0 frame, control flow proceeds to step 512. Ifthe frame is a Type-1 frame, control flow proceeds to step 520.

In step 512, for a Type-0 frame, the FCB and LPC gains are establishedfor the frame, and a check is performed to determine if the frame liesbelow the threshold curve. If the frame lies below the threshold cure,control flow proceeds to step 514 where decoding continues. If the framedoes not lie below the threshold cure, control flow proceeds to step516.

In step 516, frame erasure processing, and/or FCB gain reduction, and/ormemory state reset processing may be performed. Control flow proceeds tostep 518 where decoding continues.

In step 520, for a Type-1 frame, the previous frame is checked todetermine if the frame is an eighth rate frame, or a quarter rate frame.If the previous frame is not an eighth rate or quarter rate frame,indicating a legal rate transition, control flow proceeds to step 526where full rate decoding continues. If the previous frame is an eighth,or quarter rate frame, indicating an illegal rate transition, controlflow proceeds to step 522.

In step 522, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 524 wheredecoding continues.

FIG. 6 illustrates a method in accordance with one embodiment fordetection of rate errors in frames identified by the RDA as half rateframes. One skilled in the art will understand that the ordering ofsteps illustrated in FIG. 6 is not limiting. The method is readilyamended by omission or re-ordering of the steps illustrated withoutdeparting from the scope of the disclosed embodiments.

In step 602, the rate error detector inputs a data frame determined bythe RDA to be a half rate frame. Control flows to step 604.

In step 604, the frame is checked for frame type. If the frame is aType-0 frame, control flow proceeds to step 606. If the frame is aType-1 frame, control flow proceeds to step 614.

In step 606, for a Type-0 frame, the FCB and LPC gains are establishedfor the frame, and a check is performed to determine if the frame liesbelow the threshold curve. If the frame lies below the threshold cure,control flow proceeds to step 610 where decoding continues. If the framedoes not lie below the threshold cure, control flow proceeds to step608.

In step 608, frame erasure processing, and/or FCB gain reduction, and/ormemory state reset processing may be performed. Control flow proceeds tostep 612 where decoding continues.

In step 614, for a Type-1 frame, the previous frame is checked todetermine if the frame is an eighth rate, or quarter rate frame. If theprevious frame is not an eighth rate or quarter rate frame, indicating alegal rate transition, control flow proceeds to step 620 where half ratedecoding continues. If the previous frame is an eighth rate or quarterrate frame, indicating an illegal rate transition, control flow proceedsto step 616.

In step 616, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 618 wheredecoding continues.

FIG. 7 illustrates a method in accordance with one embodiment fordetection of rate errors in frames identified by the RDA as quarter rateframes. One skilled in the art will understand that the ordering ofsteps illustrated in FIG. 7 is not limiting. The method is readilyamended by omission or re-ordering of the steps illustrated withoutdeparting from the scope of the disclosed embodiments.

In step 702, the rate error detector inputs a data frame determined bythe RDA to be a quarter rate frame. Control flows to step 704.

In step 704, the reserved bit or sanity bit is tested to determinewhether the received value equals the fixed value set by the encoder. Ifthe bit is not equal to the fixed value set by the encoder, indicating aframe rate error, control flow proceeds to step 706. Otherwise, controlflow proceeds to step 710.

In step 706, frame erasure processing and/or memory state resetprocessing is performed. Control flow proceeds to step 708 wheredecoding continues.

In step 710, the two-bit pattern used to identify the selected shapingfilter is validated. If two-bit pattern is valid, control flow proceedsto step 716 where quarter rate decoding continues. If the two-bitpattern is not valid, control flow proceeds to step 712.

In step 712, frame erasure processing and/or memory state resetprocessing is performed. Control flow proceeds to step 714 wheredecoding continues.

FIG. 8 illustrates a method in accordance with one embodiment fordetection of rate errors in frames identified by the RDA as eighth rateframes. One skilled in the art will understand that ordering of stepsillustrated in FIG. 8 is not limiting. The method is readily amended byomission or re-ordering of the steps illustrated without departing fromthe scope of the disclosed embodiments.

In step 802, the rate error detector inputs a data frame determined bythe RDA to be an eighth rate frame. Control flows to step 804.

In step 804, the previous frame is checked to determine whether it is afull rate frame. If the previous frame is not a full rate frame,indicating a legal rate transition, control flow proceeds to step 810.If the previous frame is a full rate frame, indicating an illegal ratetransition, control flow proceeds to step 806.

In step 806, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 808 wheredecoding continues.

In step 810, the previous frame is checked to determine whether it is aquarter rate, half rate, or full rate frame. If the previous frame is aquarter rate, half rate, or full rate frame, indicating a possibleillegal rate transition, control flow proceeds to step 820. If theprevious frame is not a quarter, half, or full rate frame, indicating alegal eighth rate transition, control flow proceeds to step 812.

In step 812, the eighth rate excitation gain is compared to a maximumthreshold value. If the eighth rate excitation gain is less than thethreshold value, control flow proceeds to step 818 where eighth ratedecoding continues. If the eighth rate excitation gain is greater thanthe threshold value, indicating a rate error, control flow proceeds tostep 814.

In step 814, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 816 wheredecoding continues.

In step 820, the second previous frame is checked to determine whetherit is an eighth rate frame. If the second previous frame is not aneighth rate frame, indicating a legal rate transition, control flowproceeds to step 826. If the second previous frame is an eighth frame,indicating an illegal rate transition, control flow proceeds to step822.

In step 822, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 824 wheredecoding continues.

In step 826, the previous frame is checked to determine whether it is ahalf rate Type-1, or a full rate Type-1 frame. If the previous frame isnot a half rate Type-1, or a full rate Type-1 frame, indicating a legalrate transition, control flow proceeds to step 832. If the previousframe is a half rate Type-1 frame or a full-rate Type-1 frame,indicating an illegal rate transition, control flow proceeds to step828.

In step 828, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 830 wheredecoding continues.

In step 832, the eighth rate excitation gain is compared to a maximumthreshold value. If the eighth rate excitation gain is less than thethreshold value, control flow proceeds to step 838 where eighth ratedecoding continues. If the eighth rate excitation gain is greater thanthe threshold value, indicating a rate error, control flow proceeds tostep 834.

In step 834, frame erasure processing, and/or memory state resetprocessing may be performed. Control flow proceeds to step 836 wheredecoding continues.

FIG. 9 is a scatter plot showing the relationship between the normalizedFCB gain and the LP prediction gain. Circles below the solid curve weregenerated by clean speech, and the asterisks above the solid linescorrespond to unacceptable screeches caused by RDA errors. The solidcurve represents a threshold curve that separates the region of goodspeech from unacceptable screeches. This threshold can easily berepresented in a parametric form and incorporated into the rate errordetector.

Thus, a novel and improved method and apparatus for detection of rateerrors in variable rate receivers have been described. Those of skill inthe art would understand that the various illustrative logical blocks,modules, circuits, and algorithm steps described in connection with theembodiments disclosed herein may be implemented as electronic hardware,computer software, or combinations of both. The various illustrativecomponents, blocks, modules, circuits, and steps have been describedgenerally in terms of their functionality. Whether the functionality isimplemented as hardware or software depends upon the particularapplication and design constraints imposed on the overall system.Skilled artisans recognize the interchangeability of hardware andsoftware under these circumstances, and how best to implement thedescribed functionality for each particular application. As examples,the various illustrative logical blocks, modules, circuits, andalgorithm steps described in connection with the embodiments disclosedherein may be implemented or performed with a digital signal processor(DSP), an application specific integrated circuit (ASIC), a fieldprogrammable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components such as,e.g., registers and FIFO, a processor executing a set of firmwareinstructions, any conventional programmable software module and aprocessor, or any combination thereof. The processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine.The software module could reside in RAM memory, flash memory, ROMmemory, registers, hard disk, a removable disk, a CD-ROM, or any otherform of storage medium known in the art. Those of skill would furtherappreciate that the data, instructions, commands, information, signals,bits, symbols, and chips that may be referenced throughout the abovedescription are represented by voltages, currents, electromagneticwaves, magnetic fields or particles, optical fields or particles, or anycombination thereof.

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the disclosedembodiments. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the disclosed embodiments are not intendedto be limited to the embodiments shown herein but is to be accorded thewidest scope consistent with the principles and novel features disclosedherein.

1. A method for detecting rate errors in a variable rate receiver,comprising: receiving an encoded audio signal; extracting an encodedrate from the received encoded audio signal to provide an encoded rate;extracting an encoded reserved bit from the received encoded audiosignal to provide an encoded reserved bit; and detecting errors in theencoded rate wherein the detecting errors in the encoded rate comprisesvalidating a predetermined value of the encoded reserved bit.
 2. Themethod of claim 1 wherein the audio signal is a speech signal.
 3. Themethod of claim 2 wherein the detecting errors in the encoded ratefurther comprises: validating a set of illegal classificationtransitions of consecutive frames of the speech signal based onknowledge of speech classification and phonetic character ofconversational speech; and determining the occurrence of illegalclassification transitions.
 4. The method of claim 3 wherein the set ofillegal classification transitions comprises a full-rate frame followedby an eighth-rate frame.
 5. The method of claim 3 wherein the set ofillegal classification transitions comprises a full-rate Type-1 framefollowed by an eighth-rate frame.
 6. The method of claim 3 wherein theset of illegal classification transitions comprises a half-rate Type-1frame followed by an eighth-rate frame.
 7. The method of claim 3 whereinthe set of illegal classification transitions comprises a quarter-rateframe followed by a Type-1 full-rate frame.
 8. The method of claim 3wherein the set of illegal classification transitions comprises aquarter-rate frame followed by a Type-1 half-rate frame.
 9. The methodof claim 3 wherein the set of illegal classification transitionscomprises an eighth-rate frame followed by a Type-1 full-rate frame. 10.The method of claim 3 wherein the set of illegal classificationtransitions comprises an eighth-rate frame followed by a Type-1half-rate frame.
 11. The method of claim 3 wherein the set of illegalclassification transitions comprises an eighth-rate frame followed by aquarter-rate frame followed by an eighth-rate frame.
 12. The method ofclaim 3 wherein the set of illegal classification transitions comprisesan eighth-rate frame followed by a half-rate frame followed by aneighth-rate frame.
 13. The method of claim 3 wherein the set of illegalclassification transitions comprises an eighth-rate frame followed by afull-rate frame followed by an eighth-rate frame.
 14. The method ofclaim 1 wherein the detecting errors in the encoded rate comprises:extracting an encoded filter type identifier from the received encodedaudio signal to provide an encoded filter type identifier; andvalidating a predetermined value of the encoded filter type identifier.15. The method of claim 14 wherein the encoded filter type identifiercomprises two bits.
 16. The method of claim 15 wherein three of fourtwo-bit filter type identifier combinations identify three filter types,and one two-bit filter type identifier combination is unused.
 17. Themethod of claim 1 further comprising perceptually smoothing the effectsof detected rate errors by performing memory state reset processing. 18.The method of claim 17 wherein the memory state reset processingcomprises overwriting memory values with zeros.
 19. The method of claim17 wherein the memory state reset processing comprises overwritingmemory values with initialization values.
 20. The method of claim 17wherein the memory state reset processing is performed on moving-averagevector quantizer memory for fixed code book gain.
 21. The method ofclaim 17 wherein the memory state reset processing is performed onexcitation memory.
 22. The method of claim 17 wherein the memory statereset processing is performed on LPC synthesis memory.
 23. The method ofclaim 17 wherein the memory state reset processing is performed onpost-filter synthesis memory.
 24. A rate error detection system,comprising: means for receiving an encoded audio signal; means forextracting an encoded rate from the received encoded audio signal toprovide an encoded rate; means for extracting an encoded reserved bitfrom the received encoded audio signal to provide an encoded reservedbit; and means for detecting errors in the encoded rate wherein thedetecting errors in the encoded rate comprises validating apredetermined value of the encoded reserved bit.
 25. The error detectionsystem of claim 24 wherein the means for receiving an encoded audiosignal is a mobile subscriber unit.
 26. The error detection system ofclaim 24 wherein the means for receiving an encoded audio signal is abase station transceiver.
 27. The error detection system of claim 24wherein the audio signal is a speech signal.
 28. The error detectionsystem of claim 27 wherein the means for detecting errors in the encodedrate further comprises: means for validating a set of illegalclassification transitions of consecutive frames of the speech signalbased on knowledge of speech classification and phonetic character ofconversational speech; and means for determining the occurrence ofillegal classification transitions.
 29. The error detection system ofclaim 28 wherein the means for validating a set of illegalclassification transitions comprises means for validating an illegalclassification transition for a full-rate frame followed by aneighth-rate frame.
 30. The error detection system of claim 28 whereinthe means for validating a set of illegal classification transitionscomprises means for validating an illegal classification transition fora full-rate Type-1 frame followed by an eighth-rate frame.
 31. The errordetection system of claim 28 wherein the means for validating a set ofillegal classification transitions comprises means for validating anillegal classification transition for a half-rate Type-1 frame followedby an eighth-rate frame.
 32. The error detection system of claim 28wherein the means for validating a set of illegal classificationtransitions comprises means for validating an illegal classificationtransition for a quarter-rate frame followed by a Type-1 full-rateframe.
 33. The error detection system of claim 28 wherein the means forvalidating a set of illegal classification transitions comprises meansfor validating an illegal classification transition for a quarter-rateframe followed by a Type-1 half-rate frame.
 34. The error detectionsystem of claim 28 wherein the means for validating a set of illegalclassification transitions comprises means for validating an illegalclassification transition for an eighth-rate frame followed by a Type-1full-rate frame.
 35. The error detection system of claim 28 wherein themeans for validating a set of illegal classification transitionscomprises means for validating an illegal classification transition foran eighth-rate frame followed by a Type-1 half-rate frame.
 36. The errordetection system of claim 28 wherein the means for validating a set ofillegal classification transitions comprises means for validating anillegal classification transition for an eighth-rate frame followed by aquarter-rate frame followed by an eighth-rate frame.
 37. The errordetection system of claim 28 wherein the means for validating a set ofillegal classification transitions comprises means for validating anillegal classification transition for an eighth-rate frame followed by ahalf-rate frame followed by an eighth-rate frame.
 38. The errordetection system of claim 28 wherein the means for validating a set ofillegal classification transitions comprises means for validating anillegal classification transition for an eighth-rate frame followed by afull-rate frame followed by an eighth-rate frame.
 39. The rate errordetection system of claim 24 wherein the means for detecting errors inthe encoded rate comprises: means for extracting an encoded filter typeidentifier from the received encoded audio signal to provide an encodedfilter type identifier; and means for validating a predetermined valueof the encoded filter type identifier.
 40. The rate error detectionsystem of claim 24 further comprising means for perceptually smoothingthe effects of detected rate errors by performing memory state resetprocessing.
 41. The error detection system of claim 40 wherein the meansfor perceptually smoothing comprises means for overwriting memory valueswith zeros.
 42. The error detection system of claim 40 wherein the meansfor perceptually smoothing comprises means for overwriting memory valueswith initialization values.
 43. The error detection system of claim 40wherein the means for perceptually smoothing comprises means forperforming memory state reset processing on moving-average vectorquantizer memory for fixed code book gain.
 44. The error detectionsystem of claim 40 wherein the means for perceptually smoothingcomprises means for performing memory state reset processing onexcitation memory.
 45. The error detection system of claim 40 whereinthe means for perceptually smoothing comprises means for performingmemory state reset processing on LPC synthesis memory.
 46. The errordetection system of claim 40 wherein the means for perceptuallysmoothing comprises means for performing memory state reset processingon and post-filter synthesis memory.
 47. The error detection system ofclaim 40 wherein the means for perceptually smoothing comprises meansfor overwriting memory values with initialization values.
 48. A rateerror detection system, comprising: a receiver for receiving an encodedaudio signal; a rate extraction element for extracting an encoded ratefrom the received encoded audio signal to provide an encoded rate; a bitextraction element for extracting an encoded reserved bit from thereceived encoded audio signal to provide an encoded reserved bit; and arate error detector for detecting errors in the encoded rate wherein thedetecting errors in the encoded rate comprises validating apredetermined value of the encoded reserved bit.
 49. The rate errordetection system of claim 48 wherein the audio signal is a speechsignal.
 50. The rate error detection system of claim 49 wherein the rateerror detector further comprises an illegal classification transitiongenerator for validating a set of illegal classification transitions ofconsecutive frames of the speech signal based on knowledge of speechclassification and phonetic character of conversational speech anddetermining the occurrence of illegal classification transitions. 51.The rate error detection system of claim 48 wherein the rate errordetector comprises a rate error detector for perceptually smoothing theeffects of detected rate errors by performing memory state resetprocessing.
 52. A computer-readable medium comprising instructions thatupon execution in a processor cause the processor to perform the methodas recited in claim 1.